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Altagoan RiverLondonderry
Summary Full report
Site Type: Stream section
Site Status: PASSI
District: Magherafelt District Council
Grid Reference: H79929456, H80339300
Rock Age: Carboniferous (Tournasian)
Rock Name: Altagoan Formation, Drumard Member, Mormeal Member, Tyrone Group
Rock Type: Algal limestone, Evaporite, Limestone, Mudstone, Sand, Sandstone, Shale, Siltstone
Minerals: Calcite
Fossil Groups: Bivalve, Crustacea, Miospore
Other interest: No data, Fluvial sediments, Inter-tidal feature, Marine sediments, alluvial fan, alluvial plain, desert, sabkha

Summary of site:

This river valley locality is the best exposure of rocks of the Altagoan Formation and, although unsatisfactory in many respects, it forms the stratotype. These are some of the earliest rocks of the Carboniferous period seen in Northern Ireland. The Altagoan Formation consists of two members, the lower Drumard Member and the upper Mormeal Member. The junction with the earlier Iniscarn Formation is not seen here, and neither is the junction between the Drumard and Mormeal Members nor the top of the Mormeal Member.

The Drumard Member consists of red/grey mudstone with red micaceous sandstones. The Mormeal Member is isolated between two large faults and only a small thickness can be seen, consisting of grey shales and mudstones with occasional grey siltstones and greenish-grey sandstones. There are five levels of evaporites (rocks formed when minerals crystallized out from evaporating concentrated brines) which provide clear evidence of hot, arid land conditions. In this rock the original minerals have dissolved but the shapes of their crystals remain. This member also contains a rich flora of spores, particularly useful for dating purposes, which places it in the late Tournaisian (the first epoch of the Carboniferous period) with an age of around 350 million years. The story of these rocks is of a tropical sea advancing northwards on to a landmass. The red beds of the Drumarg Member are typical of river deposits formed on a hot, dry coastal plain (confirmed by the occasional soil layer) and these beds are almost certainly draped over earlier pebble beds and sands not seen at Altagoan. Only at the very top of the member, with the appearance of bivalves and ostracods, is there evidence of the encroaching sea. The Mormeal Member shows an arid, desolate, intertidal landscape with large areas of salt flats submerged by periodic incursions of the sea. The rich spore flora probably blew on to the flats from land to the north.

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